Importance of Clay in Iron Transport and Sediment Reddening: Evidence from Reduction Features of the Abo Formation, New Mexico, U.S.A.

Petrographic and geochemical data from reduction features in the Abo Formation (Permian) of central New Mexico provide insight into the source of iron for hematite precipitation in red beds. Reduction features in the Abo Formation include isolated reduction spots as well as zones of reduction associated with fractures and coarser-grained beds. The reduction features contain less iron and more copper than the host rock (mean values in ppm: host rock, Fe = 6456, Cu = 9; reduced Fe = 3730, Cu = 113). Hematite relics are present in reduced areas associated with fractures and reduced bedding but are absent in reduction spots. Hematite apparently never formed in the reduction spots because of localized reducing conditions present at the time of pervasive reddening. Reduced fractures and bedding most likely result from migration of reducing pore fluids flowing through more permeable areas, and postdate hematite precipitation. Iron-bearing sand and silt grains, which are typically regarded to be the principal source of iron for hematite precipitation in red beds, are uncommon (average < 1%) in the Abo Formation. Where present these grains are not significantly altered in either red host rock or in the white reduction features. Therefore, detrital iron-bearing clay-size material is the primary source of iron in the Abo Formation. Although compositionally variable, the clay-rich areas contain a mean of 8.4% FeO(total) in the host rock and a mean of 4.6% FeO(total) in reduction spots. Chemical and mineralogical data from clay-rich areas suggest that ferric oxyhydroxides associated with the clays are a more important iron source than iron from the clay crystal structure. Low whole-rock iron content of the Abo Formation relative to other red bed units is consistent with clay and associated ferric oxyhydroxides being the principal iron source. The Abo Formation may represent a clay-dominated end member in a continuum of red beds in which the iron needed for reddening is derived from clays and coarser iron bearing framework grains.